MIC2571

Micrel

MIC2571

8

1997

When the output switch turns off, the voltage across the

inductor changes sign and flies high in an attempt to maintain

a constant current. The inductor voltage will eventually be

clamped to a diode drop above V

output switch is off, the voltage across the inductor is:

V

= V

+ V

– V

2

OUT

DIODE

IN

For normal operation the inductor current is a triangular

waveform which returns to zero current (discontinuous mode)

at each cycle. At the threshold between continuous and

discontinuous operation we can use the fact that I

V

t = V

t

11

2

2

××

V

V

=

t

t

1

2

2

1

This relationship is useful for finding the desired oscillator

duty cycle based on input and output voltages. Since input

voltages typically vary widely over the life of the battery, care

must be taken to consider the worst case voltage for each

parameter. For example, the worst case for t

at its minimum value and the worst case for t

at its maximum value (assuming that V

do not change much).

To select an inductor for a particular application, the worst

case input and output conditions must be determined. Based

on the worst case output current we can estimate efficiency

and therefore the required input current. Remember that this

is

power conversion, so the worst case average input current

will occur at maximum output current and minimum input

voltage.

Average I

=

V

I

V

Efficiency

IN(max)

OUT

OUT(max)

IN(min)

×

×

Referring to Figure 1, it can be seen the peak input current will

be twice the average input current. Rearranging the inductor

equation to solve for L:

L =

V

I

t

1

×

L =

V

2

Average I

t

IN(min)

IN(max)

1

×

×

where t =

duty cycle

f

1

OSC

To illustrate the use of these equations a design example will

be given:

Assume:

MIC2571-1 (fixed oscillator)

V

I

V

efficiency = 75%.

Average I

=

5V

5mA

1.0V

0.75

= 33.3mA

IN(max)

×

×

L =

1.0V

0.7

2

33.3mA

20kHz

×

××

L = 525

µH

Use the next lowest standard value of inductor and verify that

it does not saturate at a current below about 75mA

(< 2

× 33.3mA).